In each of the expandable tire building machines shown in U. S. Pat. Nos. 3,171,769 to Henley et al. and 3,475,254 to Henley, the building drum is made of radially movable segments, numbered 59 in the '769 patent and 116 in the '254 patent. These segments are movable radially outwardly and inwardly to change the diameter of the drum. This allows the carcass plies to be wrapped around the drum with the main body of the drum at approximately the same diameter as the turnover bladders. Then the drum can be expanded to allow the beads to be set adjacent the ends of the drum and above the turnover bladders, as shown in FIGS. 9 and 10 of the '769 patent. To move the expandable drum segments radially outwardly, there are inflatable bladders provided beneath the drum segments, these bladders being identified by the numeral 58 in the '769 patent and the numeral 111 in the '254 patent. Both of these bladders have torus-shaped configurations and radially inwardly facing stems through which air is introduced to inflate the bladders, as shown in FIG. 6 of the '769 patent and FIG. 9 of the '254 patent.
One problem with torus-shaped bladders such as those shown in the foregoing patents is that, as such bladders expand after their initial contact with the drum segments, their area of contact with the drum segments diminishes. This is because such a bladder expands simply by its initially cylindrical walls stretching into arcuate bulges. As the outer wall of the bladder bulges, it becomes rounder, with less and less of its area in contact with the drum segments it presses against. As this area of contact decreases, the bladder exerts less and less force against the drum segments in proportion to the pressure in the bladder. This condition, added to the drum segments being biased radially inwardly by springs, results in the need for a very high bladder pressure to push the drum segments radially outwardly and to hold them in their radially outward positions. The bladder pressure must be sufficient to withstand the opposing forces of the carcass ply turnover bladders as they roll over the side portions of the expanded drum, and the opposing forces of the stitcher wheels as they roll over the drum to stitch the carcass ply ends to the main body of the carcass. The requirement for such a high bladder pressure means that the bladder walls must endure great stresses and are subject to great fatigue. Also, even small air leaks in the bladder inflation system become critical, resulting in frequent failures of the drum to expand properly and to remain in its expanded condition when the carcass ply ends are being secured around the bead rings.
The foregoing problems might be alleviated in some cases by using larger bladders that initially have a larger surface contact area with the drum segments. With such larger bladders, the loss in contact area upon full inflation of the bladder would not be as critical. However, in many cases there is not enough space along the length of the drum to use a larger bladder. This is particularly true with drums for building radial tires such as that shown in the '254 Henley patent referred to above. With such a radial tube building drum, the side portions of the drum must be capable of being moved axially together when the carcass is expanded to its toroidal shape, as shown in FIG. 30 of that patent. This seriously limits the space for containing the bladders that push against the drum segments to expand the drum, and thus, larger bladders of the conventional construction are not feasible.